"1st argument ('__restrict int') would lose restrict qualifier" when passing (this restrict *)->int to int& argument

[nabijaczleweli]

void f(int& x) {
    (void)x;
}

class C {
    int x;
    void g() __restrict;
};

void C::g() __restrict {
    f(this->x);
}

(same for void f(int const& x)) compiles fine on GCC, but Clang 17 and 19.0.0 (++20240212105217+93cdd1b5cfa3-1~exp1~20240212225341.524) rejects it with

a.cpp:11:5: error: no matching function for call to 'f'
   11 |     f(this->x);
      |     ^
a.cpp:1:6: note: candidate function not viable: 1st argument ('__restrict int') would lose restrict qualifier
    1 | void f(int& x) {
      |      ^ ~~~~~~
1 error generated.

I don't think this is valid – this looks like this is of type C restrict *, and thus x is compatible with int restrict &, and it's treating restrict like a cv-qualifier, which I don't think it is (and isn't for pointers), and even if it were I haven't found a position where you could make f take an int restrict &.

[zopsicle]

this looks like this is of type C restrict *

The following example confirms this:

class C
{
    void g() __restrict;
};

void C::g() __restrict
{
    double x = this;
}

fails with:

error: cannot initialize a variable of type 'double' with an rvalue of type '__restrict C *'
    8 |     double x = this;
      |            ^   ~~~~

note that the diagnostic says __restrict C *, not C * __restrict. Compiling the same program with GCC fails with:

error: cannot convert 'C* __restrict__' to 'double' in initialization
    8 |     double x = this;
      |                ^~~~
      |                |
      |                C* __restrict__

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: наб (nabijaczleweli)

```cpp void f(int& x) { (void)x; }

class C {
int x;
void g() __restrict;
};

void C::g() __restrict {
f(this->x);
}

(same for `void f(int const& x)`) compiles fine on GCC, but Clang 17 and 19.0.0 (++20240212105217+93cdd1b5cfa3-1\~exp1\~20240212225341.524) rejects it with

a.cpp:11:5: error: no matching function for call to 'f'
11 | f(this->x);
| ^
a.cpp:1:6: note: candidate function not viable: 1st argument ('__restrict int') would lose restrict qualifier
1 | void f(int& x) {
| ^ ~~~~~~
1 error generated.

I don't think this is valid – this looks like `this` is of type `C restrict *`, and thus `x` is compatible with `int restrict &`, and it's treating restrict like a cv-qualifier, which I don't think it is (and isn't for pointers), and even if it were I haven't found a position where you *could* make f take an `int restrict &`.
</details>

[shafik]

Based on gcc documentation: https://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html

it does not look like we are doing the right thing here but not sure.

CC @erichkeane @AaronBallman

[AaronBallman]

Yeah, this AST looks wrong to me: https://godbolt.org/z/4P7a45dP1 specifically:

`-CXXThisExpr <col:11> '__restrict S *' this

That should be a restricted pointer, not a restricted pointee.

[Sirraide]

I think the issue seems to be that we treat __restrict like const and volatile, but whereas the latter are applied to the pointee, the former should be applied to the pointer; I’m currently working on a fix for this